1. Field of the Invention
The present invention relates to a semiconductor imaging device having a plurality of pixels arranged in a matrix-like pattern.
2. Description of the Related Art
Generally, semiconductor imaging elements are classified broadly into CCD image sensors and CMOS image sensors. As one type of CMOS image sensors, there exists a 4Tr+1PD type 4Tr-APS (Active Pixel Sensor) including four transistors and one photodiode (PD) which form a single pixel.
The structure of a pixel composing the 4Tr-APS is shown in FIGS. 24A and 24B.
In the 4Tr-APS, as shown in FIG. 24A, a photodiode (PD) photoelectrically converts the received light by means of NP junction and accumulates a generated signal charge (electron) in an N-type diffusion layer. Floating diffusion (FD) layer constitutes a capacitance for converting the signal charge transferred from the PD into voltage. A transfer gate transistor (TG-Tr) controls the transferring of the signal charge from the PD to the FD. A reset transistor (RS-Tr) resets the voltage of the FD to a reset voltage Vr. A source follower transistor (SF-Tr) outputs a signal voltage according to the voltage of the FD (varies with the signal charge). A select transistor (SL-Tr) exists for selecting the row to which the pixel belongs.
In each of the pixels, as shown in FIG. 24B, a TG line connected to the gate electrode of the TG-Tr, an RS line connected to the gate electrode of the RS-Tr, and an SL line connected to the gate electrode of the SL-Tr are respectively provided in alignment along the row direction, while a Vr line providing reset voltage Vr and an SG line connected to the source electrode of the SF-Tr for reading the signal voltage are respectively provided in alignment along the column direction. Each of the pixels is selected per row by the TG, SL, and RS lines to read the signal voltage.
An exemplary operation sequence of the 4Tr-APS is shown in FIG. 25.
When a row is selected (SL-Tr turned ON), the RST-Tr first turns ON with the TG-Tr remaining OFF, then the FD is reset, and a signal voltage according to the voltage of the FD is read as noise (N). Next, following the ON/OFF switching of the TG-Tr, the signal charge is transferred from the PD to the FD. In the FD, the voltage drops by an extent corresponding to the signal charge transferred from the PD, and signal voltage according to the voltage is output from SF-Tr as a signal (S).
Today, there is a rising demand for downsizing/miniaturization of 4Tr-APS too, with proposals being made for sharing elements between adjacent pixels.
For example, Patent Document 1 discloses a technique of sharing a signal voltage reading unit (composed of an SF-Tr, an RS-Tr, and an SL-Tr) between 2*2 adjacent PDs and 2*4 PDs.
In addition, Patent Document 2 discloses a technique of sharing the signal voltage reading unit between two PDs along the column direction and a technique of sharing the signal voltage reading unit between four PDs along the column direction. In the latter case, specifically, the four PDs (PD1 to PD4) which are adjacent along the column direction are disposed in the order of PD1/FD1/PD2/signal voltage reading unit/PD3/FD2/PD4.
[Patent Document 1] Japanese Patent Application Laid-open No. 2000-232216
[Patent Document 2] Japanese Patent Application Laid-open No. 2001-298177
However, the conventional art stated above has serious problems as described below.
With Patent Document 1, since the transistors of the signal voltage reading unit are disposed between adjacent PDs along the column direction as well as between adjacent PDs along the row direction for respective PDs, they occupy a considerably large area as a pixel array. Therefore, in this case, the 4Tr-APS cannot be sufficiently downsized/miniaturized.
Thus, expectations are arising for a technique for sharing the signal voltage reading unit between four PDs which are adjacent along the column direction. However, in the case of Patent Document 2, sufficient downsizing cannot be achieved, in the first place, according to the technique for sharing the signal voltage reading unit between two PDs along the column direction. Additionally, with the technique for sharing a signal voltage reading unit between four PDs along the column direction, although it is not necessary to dispose the transistor of the signal voltage reading unit between PDs which are adjacent along the row direction, there may be a portion where the PDs are unevenly disposed in terms of a pixel array. In this case, respective PDs may have different light receiving conditions, making correct imaging operation difficult. In addition, a problem arises in that signal voltage variation may occur because the signal voltage reading unit is disposed between adjacent PDs and necessarily, the area to be occupied by the signal voltage reading unit must be reduced.